The present invention relates generally to 2,2'-bipyridyl compounds, and more particularly to an improved process for the preparation of 2,2'-bipyridyls which is characterized by the novel use of a supported elemental nickel catalyst and methods for reactivating and for extending the useful life of the same.
2,2'-Bipyridyl compounds are among the most useful of pyridine derivatives. They have demonstrated significant economic and technological importance as chelating agents and paint additives and in improved coating compositions. See G. M. Badger & W. H. F. Sasse, Adv. Heterocyclic Chem. 2, 179 (1963) and U.S. Pat. No. 2,526,718 to G. K. Wheeler. They are also widely used to form bridged quaternary salts, commonly known as Diquats which have important herbicidal properties. See L. A. Summers, "The Bipyridinium Herbicides," Academic Press (1980).
Although other routes to 2,2'-bipyridyls have been reported, one of the more advantageous syntheses has been the action of a Raney nickel metal catalyst on pyridine at elevated temperatures. The importance of this catalyst for other reactions harkens back to the early work of Dr. Murray Raney, who originally described the formation and use of a skeletal nickel prepared by treating a nickel aluminum alloy ("Raney alloy") with a caustic material such as sodium hydroxide to leach out the aluminum metal. The resulting skeletal nickel structure has a high surface area compared to other nickel forms, and has been the catalyst of choice for the preparation of 2,2'-bipyridyls from pyridine bases. A primary focus in this area has in turn been to characterize the most preferred ratio of nickel to aluminum in the Raney alloy used and to develop devices and techniques to optimize the conditions and use of the Raney nickel catalyst once made. See, e.g., U.S. Pat. No. 3,822,279 to Joy et al.; Chem. Abstr. Vol. 78, 111132 z (1973); U.S. Pat. No. 3,053,846 to Varcoe; and Chem. Abstr. Vol. 70, 3770 g (1969).
Despite its long-standing predominance, Raney nickel and the processes for its use have also demonstrated significant disadvantages. For example, Raney nickel is a fine powder or a wet sludge which severely complicates its handling and use. Early-on, the 2,2'-bipyridyl reaction was carried out by simply refluxing pyridine on the Raney nickel. It was then discovered that this led to rapid deactivation of the catalyst which has been attributed by some to this prolonged contact with the 2,2'-bipyridyl product itself. See Badger & Sasse, supra, p. 199 which reports that "[t]he formation of 2,2'-bipyridine ceases after 50 hours of refluxing." and U.S. Pat. No. 3,053,846 to Varcoe. In later work, alternative reaction schemes and equipment were developed, often focusing on some means whereby pyridine vapors are condensed over and then the resulting hot liquid allowed to stand in contact with the Raney nickel catalyst and slowly filter through the bed in a fashion somewhat similar to a common Soxhlet extractor. This method was thought an improvement because the newly condensing pyridine liquid was intended to displace the previous reaction products in hopes of shortening the period over which the catalyst remains in contact with the 2,2'-bipyridyl formed. Compare, U.S. Pat. No. 3,053,846 which reports a recovery of 0.0231 g of 2,2'-bipyridyl/g Ni/h against W. H. F. Sasse, Org. Syn. Coll., 5, p. 102 (1973) which reports only 0.007 g of 2,2'-bipyridyl produced per g of nickel catalyst per hour. However, while such later processes are feasible, their equipment is often difficult and expensive to build and operate especially when attempting to scale up from the laboratory to the industrial level. Additionally, notwithstanding the many efforts to optimize the use of Raney nickel catalyst, such reactions have demonstrated relatively low rates of conversion to 2,2'-bipyridyls.
In addition to these difficulties, Raney nickel with its skeletal structure is well known to be extremely pyrophoric which presents significant other problems in processing and handling. See R. Habermehl, Chem. Eng. Progress, February 1988, pp. 16-19. Moreover, the caustic substances used to treat Raney alloy in forming the catalyst are commonly aqueous solutions. The Raney nickel is thereby formed as an aqueous paste from which the water must be removed for use in this and many other reactions. This step has proven to be particularly dangerous and expensive, and has been the subject of several patents in the field. For example, the Sasse article in Org. Syn. Coll., 5, reported drying Raney nickel catalyst at 25.degree.-30.degree. C. under partial vacuum (17-20 mm) over 4-12 hours while emphasizing caution and noting that when heated under vacuum, Raney nickel catalyst may suddenly give off large quantities of heat and hydrogen resulting in a dangerous explosion. See also, U.S. Pat. No. 3,152,137 to Lang et al. (and corresponding British Patent No. 899,015); and British Patent No. 948,956. The production of Raney alloy itself also involves a costly process requiring extreme conditions, see U.S. Pat. No. 3,822,279 to Joy et al., and disposal of spent Raney nickel catalyst is complicated because it has been identified as a carcinogen. See A. Agoos, Chemical Week, Dec. 10, 1986, pp. 44-47.
Another prevalent problem has been that Raney nickel catalyst tends to suffer a significant loss of activity as the reaction proceeds. Those skilled in this field have yet to discover an effective way to reactivate such a catalyst once it has lost part or all of its activity. One method reported to have some success is to wash the spent Raney nickel with an alcoholic solution of an alkali metal hydroxide such as KOH. See British Patent No. 1,202,711. Still further, although not addressing reactivation, the initial activation of such pyrophoric catalysts was generally discussed in U.S. Pat. No. 3,560,404 to Jung et al. which introduced the catalyst into a non-aqueous organic liquid and in contact with reducing agents such as hydrazine, borohydrides, or hypophosphites until the development of gaseous hydrogen occurred.
The mechanism of this deactivation of Raney nickel through use is not well understood, although as previously discussed some attribute it to prolonged contact of the catalyst with the 2,2'-bipyridyl product. Still other speculation has been that deactivation may be attributed to oxidation of the Raney nickel itself. In any event, deactivation of such catalysts has been a major concern in the field.
Although this background has concentrated thus far on Raney nickel, other types of catalysts have also been reported in the art for the formation of 2,2'-bipyridyls from pyridine bases. Contrary to any perception that oxidation of metal sites deactivates such catalysts, it is interesting to note that British Patent No. 1,377,213 reported the use of various metal oxides including nickel oxide in the formation of 2,2'-bipyridyls. However, the rates of formation for such 2,2'-bipyridyls were low even when the oxides were supported on an inert support and subjected to temperatures and pressures as high as 340.degree. C. and 800 psig. Similarly extreme conditions were used by two researchers who reported obtaining 2,2'-bipyridyl by heating pyridine in the presence of a nickel-alumina catalyst in an autoclave at 320.degree. C. to 325.degree. C. and at pressures of 42 to 44 atmospheres. J. P. Wibaut and H. D. Tjeenk Willink, "A Method of Synthesis of 2,2'-Dipyridyl by Catalytic Dehydrogenation of Pyridine Under Pressure," Recueil des Travaux Chimiques des Pays-Bas, Vol. 50, 1931, pp. 287-290. Still other reported catalysts include: Ziegler catalysts (U.S. Pat. No. 3,697,534 to Waddan et al.); salts of certain noble metals (British Patent No. 1,014,076 and Badger & Sasse, Adv. Heterocyclic Chem. 2 (1963)); mixtures of nickel and aluminum (Chem. Abstr. Vol. 94, 208723M (1981) and Chem Abstr. Vol. 99, 139784W (1983)); a Grignard reagent (British Patent No. 1,060,661); and certain metal or metal alloy bodies impregnated with an alkaline solution of reducing agent (British Patent No. 1,009,895). All of these have been reported with varying low levels of success.
It is in light of this extensive background that the applicants entered their study in an attempt to discover an improved process and catalyst for the preparation of 2,2'-bipyridyls which are more convenient and effective from all aspects than the prior art Raney nickel and other processes.